1. Field of the Invention
This invention pertains generally to imaging systems and methods such as computed tomography, and more particularly to methods and devices for measuring the half-value level (HVL) for imaging.
2. Description of Related Art
X-ray based imaging and other types of radiographic analysis are established diagnostic tools. The penetrating ability of photons is one of the characteristics of x-ray and gamma ray radiations that make them useful for medical imaging, for example. Photons directed at an object either completely penetrate the object or are absorbed or scattered. The amount of penetration depends on the energy of the individual photons and the atomic number, density, and thickness of the object that is being bombarded. Therefore, photon penetration can be expressed as the fraction of radiation that passes through the object and it is the inverse of attenuation.
X-ray beams used for medical purposes generally are polyenergetic i.e. the x-ray beam is comprised of a spectrum of photons with different x-ray energies. When the applied voltage (kV) between the cathode and anode in an x-ray tube is changed, the spectrum will change. The addition of metallic filters in the x-ray tube housing (or collimation structure) also changes the shape of the spectrum. Accordingly, an x-ray beam is made up of different photon energies and photons of certain energies will penetrate better than others. The low-energy photons in the x-ray spectrum will not contribute to image formation and will be absorbed or scattered by the tissue of the patient. This selective thinning of photons based on their energy by a material is referred to as filtration.
Because it is difficult to measure the actual x-ray spectrum due to the time and equipment requirements of x-ray spectroscopy, a typical metric that is used to characterize the penetrability of an x-ray beam is called the half value layer (HVL). The half-value layer (HVL) is a quantification of the penetration of the x-ray beam through a material that is being examined. HVL is defined by the thickness of a material that reduces the transmission of the x-ray beam by 50%. The units used to express the HVL are typically millimeters or centimeters and the HVL value is photon energy dependent.
Photon absorbing and filtering materials can preferentially absorb lower-energy x-ray photons emitted by the x-ray source and thereby filter the beam. The amount of filtration of the x-ray beam will depend on the voltage potential (keV) used to produce the beam and the thickness and atomic number of filter material.
Filters used in industrial radiography to filter the x-ray beam are typically made from high atomic number materials such as copper, tin, brass or lead. However, filters for medical radiography are normally made from aluminum. In diagnostic radiology, typically sheets of 1.0 mm aluminum are used as filters, but the concept is valid for any material such as copper or tin. When both the applied voltage (kV) and the HVL of a spectrum are known, an x-ray spectrum is well characterized. Most x-ray systems used for medical imaging have the ability to adjust the kV, and thus it is common practice amongst medical physicists to characterize the x-ray system by measuring the HVL at three or more kV settings.
The practice of measuring the HVL, in general, proceeds with a radiation meter (which measures x-ray beam exposure in the units of roentgens or air kerma in the units of mGy) that is positioned to measure a collimated (narrow) beam of x-rays from the source, and serial measurements are made.
The thickness of the aluminum sheets is changed between measurements by adding an additional sheet of Al. Depending on the x-ray spectrum, the beam will be attenuated differently by the added aluminum filters. HVL values typically run from about 0.25 mm of Al in mammography to 9 to 12 mm Al in computed tomography (CT) at high kV.
The half value layer is defined as the thickness of added material (aluminum is the most typical, however this material can be any metal or compound) which results in the attenuation of the initial x-ray beam (when t=0) to 50%. Mathematically, the HVL can be defined as:
  0.500  =                    ∫                  E          =                      E            ⁢                                                  ⁢            min                                    E          ⁢                                          ⁢          max                    ⁢                        k          ⁡                      (            E            )                          ⁢                                  ⁢                  Φ          ⁡                      (            E            )                          ⁢                                  ⁢                  ⅇ                                    -                                                μ                  z                                ⁡                                  (                  E                  )                                                      ×            HVL                          ⁢                                  ⁢                  ⅆ          E                                    ∫                  E          =                      E            ⁢                                                  ⁢            min                                    E          ⁢                                          ⁢          max                    ⁢                        k          ⁡                      (            E            )                          ⁢                                  ⁢                  Φ          ⁡                      (            E            )                          ⁢                                  ⁢                  ⅆ          E                    
The HVL is measured by medical physicists for x-ray beams including fluoroscopy, radiography, mammography, and in computed tomography (CT). Calculation of the HVL of an x-ray based system is usually performed for two related purposes: 1) apparatus quality assurance; and 2) radiation dose reduction in imaged patients.
Image quality is essential to a proper diagnosis and requires that the x-ray generator work under reproducible conditions. Because low energy photons do not contribute to the formation of the image and rather contribute only to the radiation dose of the patient, diagnostic x-ray equipment includes a minimum filter so that the apparatus produces a beam of a standard quality.
Since ionizing radiation can cause damage to tissues, minimizing x-ray exposure reduces the risk of injury to the patient. Therefore, consistent x-ray beam characteristics minimize the risk of excess exposure.
HVL values may also be used as a preventive indication of equipment malfunctions. Many states in the United States require HVL determinations of an x-ray beam as a standard quality assurance test for radiographic and fluoroscopic systems. If a system is not in compliance with the expected standards, corrective action may need to be taken and servicing of the equipment performed.
In addition to its utility as a quality assurance tool, the HVL of an x-ray based system is a useful indicator of the relative “hardness” of the x-ray beam. Along with the known x-ray tube voltage (kV), the HVL is a useful metric which is used extensively in the field of medical physics to describe the “hardness” of an x-ray beam. Higher energy x-ray beams are considered “hard” and penetrate objects (such as the patient) more readily than softer (lower energy) x-ray beams. HVL is also an indicator of how much soft radiation is present in the beam. Soft radiation is absorbed in the surface tissue and does not contribute to formation of the image. Image quality can be maximized and the radiation dose minimized by HVL monitoring and device calibration.